A linear or two-dimensional ion-processing device such as an ion trap is formed by a set of elongated electrodes coaxially arranged about a main or central axis of the device. Typically, each electrode is positioned in the plane (e.g., the x-y plane) orthogonal to the central axis (e.g., the z-axis) at a radial distance from the central axis. Each electrode is elongated in the sense that its dominant dimension (e.g., length) extends as a rod in parallel with the central axis. The resulting arrangement of electrodes defines an elongated interior space or chamber of the device between the inside surfaces of the electrodes that face inwardly toward the central axis. In operation, ions may be introduced, trapped, stored, isolated, and subjected to various reactions in the interior space, and may be ejected from the interior space for detection. Such manipulations require precise control over the motions of ions present in the interior space, as well as over the geometry, fabrication and assembly of the physical components of the electrode structure. The radial (or transverse) excursions of ions along the x-y plane may be controlled through application of appropriate RF signals to one or more of the electrodes to generate a two-dimensional (x-y), radial trapping field. The axial excursions of ions, or the motion of ions along the central axis, may be controlled through the application of appropriate DC signals to the electrodes to produce an axial (z) trapping field.
Additional RF signals may be applied between two opposing electrodes positioned on a radial (x or y) axis of the electrode set to produce an auxiliary or supplemental RF field that influences the motions of ions by increasing the amplitudes of their oscillations and thus increasing their kinetic energies along the radial axis as a result of resonant excitation. This type of resonant excitation along a radial direction is typically employed to eject ions from the electrode set to detect the ejected ions, or to eliminate the ejected ions so as to isolate other ions in the electrode set. The theory, mechanisms, and techniques of resonant excitation are well known to persons skilled in the art and thus need not be described in detail in the present disclosure. Briefly, excitation of an ion of a given mass-to-charge ratio occurs when the frequency of the supplemental RF field matches the secular frequency of the ion associated with motion along the axis of the dipole. If enough power is applied with the supplemental RF signal, the ion overcomes the restoring force imparted by the trapping field and is ejected from the linear ion trap in a direction along the radial axis. For this purpose, at least one of the electrodes to which the resonant dipole is applied typically includes a slot through which ejected ions can travel to an ion detector.
Resonant excitation along a radial or transverse direction may also be employed to promote collision-induced dissociation (CID). Processes involving CID are well-known in the field of tandem mass spectrometry and multi-stage mass spectrometry (MS/MS and MSn). Briefly, to effect CID, a suitable inert gas is provided in the interior space of the electrode set and collisions occur between the precursor ion and components (atoms or molecules) of the surrounding gas. The increase in kinetic energy provided by the resonant dipole enables the precursor ion to dissociate into product ions in response to at least some of these collisions. The ions can then be mass-analyzed, and/or the product ions can be isolated and the process of CID repeated for successive generations of product ions.
It is known that if too much resonant voltage is applied to the two opposing electrodes during the CID process, the ions will gain too much energy in the transverse direction. As a potential result, the amplitudes of oscillation of the ions in the transverse direction will increase until the ions strike the electrodes or are ejected through a slot in the electrode and thus are lost. The need to avoid this event limits the maximum kinetic energy that the ions may have for CID. It is also known that the RF trapping potential in the transverse direction increases with the amplitude of the RF trapping voltage applied to the electrodes and decreases with ion mass. For a given transverse trapping potential, the maximum kinetic energy available for CID is determined. Although the amplitude of the RF trapping voltage could be increased to increase the RF trapping potential, increasing the RF trapping potential also limits the mass range of ions that can be trapped in the electrode set by increasing the mass cut-off limit, thus limiting the mass range of the product ions formed by CID. Accordingly, a method of increasing the kinetic energy available for CID is needed that does not compromise the mass range.
In addition to time sequence-based devices such as multi-pole ion traps, sequential analyzer-based devices such as triple-quadrupole mass spectrometers are also employed for CID. In a triple-quadrupole mass spectrometer, the first quadrupole electrode set is utilized as a mass filter to select precursor ions, the second quadrupole electrode set is utilized as a collision cell for CID, and the third quadrupole electrode set is utilized as a mass filter to select product ions produced in the collision cell. Mass-selected precursor ions emitted from the first mass filter are accelerated to a desired energy and enter the gas-filled collision cell. The ions make one pass from the entrance to the exit of the collision cell. As the ions travel through the collision cell, collisions between the high-energy ions and the gas cause CID. The resulting product ions formed in the collision cell have sufficient kinetic energy remaining that these ions travel to the exit of the collision cell and enter the second mass filter for mass analysis. Any of the original precursor ions that have not collided will also exit the collision cell without any further opportunity to be dissociated. This well-known disadvantage of sequential analyzer-based devices limits the efficiency of converting the precursor ions into product ions by CID.
In view of the foregoing, it would be advantageous to provide techniques for increasing the maximum amount of kinetic energy attainable by ions in a linear ion-processing device such as a linear ion trap. It would also be advantageous to provide techniques for CID that increase the maximum amount of kinetic energy available for CID without limiting mass range. It would also be advantageous to provide techniques that do not rely on excitation in a direction that is radial or transverse to the central axis of a linear device. It would also be advantageous to provide techniques that do not rely on excitation by a resonant RF field. It would also be advantageous to provide techniques for CID that enable multiple cycles of trapping, excitation and dissociating the ions to increase the efficiency of the conversion of precursor ions to product ions by repeating these cycles multiple times.